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| Boilers and HRSGs


Excess air reduction and boiler cleaning: key issues for waste-to-energy plants


Figure 1. Typical process scheme for a waste-to-energy plant (source: Von Roll Inova)


Waste reception and storage 1 Unloading hall 2 Waste pit 3 Waste crane 4 Control room


Combustion, boiler, energy output 5 Feed hopper 6 Ram feeder 7 Reciprocating incineration grate 8 Bottom ash extractor 9 Bottom ash conveyor 10 Primary air supply  


  recirculation injection 13 Auxiliary burner 14 Four-pass boiler 15 Boiler drum 16 Turbine generator set 17 Air condenser


Flue gas cleaning 18 SNCR deNOx


injection levels


19 Semi-dry reactor   21 Induced draft fan 22 Stack


Consumables and residues 23 Boiler ash conveying system 24 Residue conveying system 25 Feed water tank 26 Silo for hydrated lime 27 Residue silo


Modern waste-to-energy (WtE) plants can typically achieve electrical efficiencies in the range of 28-35%, if they do not deliver process steam or district heating. About 2.5-4% of the gross generated electrical power is consumed by electric motors.


Referring to Figure 1, among the most significant electrical consumers in a typical WtE plant are the primary and secondary air fans, (10) and (11), and, most important of all, the induced draft fan (21). The power consumption of these fans, and therefore the overall electrical plant efficiency, is strongly influenced by the level of excess air in the combustion process.


. The fans were operated at full motor load and the control was achieved by dampers. Combustion control was based on a limited number of slow sensors.


Excess air, which constitutes a significant part of the flue gas volume, is necessary to enable sufficient burn out of exhaust gas as well as of solid residues (ie, slag and fly ash). In the 1950s to 1970s WtE plants were operated with an excess air level of more than 12 vol% O2


In the 1980s, the typical excess air level was reduced to around 8 vol% O2


, which led


to a reduction in energy consumption, further reduced thanks to the application of frequency converters.


Figure 2. SPGr, the latest version of Explosion Power’s Shock Pulse Generator technology (source: Explosion Power)


, with a flue gas retention time of 2 seconds above 850°C. This was based on emissions related considerations to achieve high burn out rate of organic waste gas components with state-of-the art combustion control. Nowadays, modern sensor technology for combustion control and optimised combustion chamber geometry enable excess air to be reduced to as little as 2.5 vol% O2


in order


to minimise fan power consumption and to maximise boiler efficiency, while maintaining flue gas quality.


In the 1990s, when the German “TA Luft” came into force, excess air was reduced to a minimum of 6 vol% O2


The reduction in flue gas volume will also reduce the amount of fly ash in the boiler. On the other hand, changes in waste composition can cause boiler deposits which are more difficult to remove from heating surfaces.


Maintaining a “clean boiler” will always be the key means for keeping a low flue gas pressure drop and high heat transfer. Indeed, despite the aforementioned improvements in combustion design, a boiler can only be operated successfully with an effective state-of-the-art cleaning system. In autumn 2020, Explosion Power GmbH launched the SPGr series (Figure 2). This new version of its Shock Pulse Generator (SPG) produces shock pulses by isochoric combustion of compressed air and natural gas, whereas the EG10 series uses natural gas and oxygen. Both SPG series clean the boiler on-line and automatically and can be applied from the highest flue gas temperature in the furnace to the lowest flue gas temperature in economisers or air heaters, working as well for membrane walls as for tube bundles. SPGr offers longer maintenance intervals and reduced maintenance costs. Orders for more than 80 SPGr units have already been received, thus representing a rapidly growing share among the more than 900 Shock Pulse Generators delivered worldwide for a wide range of boiler capacities and fuels.


Biofuelled tequila: Hurst calls the shots


Among sustainability initiatives taken by 140 year old Mexican tequila producer Casa Herradura that are proving highly effective was the installation of an energy-from-waste system. It employs an innovative Hurst hybrid biomass steam boiler fuelled with either biogas from a biological anaerobic reactor (part of a wastewater treatment plant) or by burning the organic agave waste produced via the tequila production process. The steam is used in the distillation process or for cogeneration.


The 800 HP boiler, which can be remote monitored 24/7 at the Hurst plant in Coolidge, GA, USA, was custom engineered and designed for the Herradura process. It incorporates a grinder, 3-pass stainless steel dryer and material conveyor system to deliver the processed agave waste to the combustion chamber. Herradura was able to realise fuel savings of up to 70% by switching to biogas/biomass fuels, and met its projected ROI on the boiler project in 13 months.


Hurst biomass boiler at Casa Herradura www.modernpowersystems.com | April 2022 | 33


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